Queues — FIFO, Deque and Priority Queue

import java.util.Queue;
import java.util.ArrayDeque;
 
Queue<Integer> queue = new ArrayDeque<>();
queue.offer(10);    // enqueue — O(1)
queue.offer(20);
queue.offer(30);
queue.poll();       // dequeue — O(1), returns 10

public List<Integer> bfs(Map<Integer, List<Integer>> graph, int start) {
    Set<Integer> visited = new HashSet<>();
    visited.add(start);
    Queue<Integer> queue = new ArrayDeque<>();
    queue.offer(start);
    List<Integer> order = new ArrayList<>();
 
    while (!queue.isEmpty()) {
        int node = queue.poll();
        order.add(node);
        for (int neighbor : graph.getOrDefault(node, new ArrayList<>())) {

import java.util.Deque;
import java.util.ArrayDeque;
 
Deque<Integer> dq = new ArrayDeque<>();
dq.offerLast(1);    // add to right
dq.offerFirst(0);   // add to left
dq.pollLast();      // remove from right
dq.pollFirst();     // remove from left

public int[] maxSlidingWindow(int[] arr, int k) {
    Deque<Integer> dq = new ArrayDeque<>();   // stores indices, decreasing order of values
    int[] result = new int[arr.length - k + 1];
    int resIdx = 0;
 
    for (int i = 0; i < arr.length; i++) {
        // Remove elements outside window
        while (!dq.isEmpty() && dq.peekFirst() < i - k + 1) {
            dq.pollFirst();
        }

import java.util.PriorityQueue;
import java.util.Collections;
 
// Min heap (default in Java)
PriorityQueue<Integer> pq = new PriorityQueue<>();
pq.offer(3);
pq.offer(1);
pq.offer(2);
pq.poll();   // returns 1 (smallest)
 
// Max heap — pass a custom comparator
PriorityQueue<Integer> maxPq = new PriorityQueue<>(Collections.reverseOrder());
maxPq.offer(3);
maxPq.offer(1);
maxPq.poll();  // returns 3 (largest)

public int[] topKFrequent(int[] nums, int k) {
    Map<Integer, Integer> count = new HashMap<>();
    for (int n : nums) {
        count.put(n, count.getOrDefault(n, 0) + 1);
    }
 
    // Min heap of size k
    PriorityQueue<Integer> heap = new PriorityQueue<>(
        (a, b) -> count.get(a) - count.get(b)
    );
    
    for (int num : count.keySet()) {
        heap.offer